Grounding of an electrical power system affects such major aspects of its operation as personnel and equipment safety, magnitude of ground fault currents, overvoltages with respect to ground, ground fault detection and isolation, and transient characteristics under ground faults. Typical applications include e.g. subway trains, locomotive distribution, hybrid vehicles, and ships. Grounding methods include an ungrounded system, a solidly grounded system, a low resistance grounded system, a reactance grounded system, and a high resistance grounded system.
Grounding systems have to maintain voltage balance with respect to ground without creating excessive power dissipation in the grounding components, ensure suppression of voltage transients resulting from ground fault and common-mode voltage spikes, and keep currents circulating in the system at a minimum. None of the known grounding methods satisfy often conflicting demands facing electrical systems, particularly such systems that comprise both AC and DC voltage distributions.
The only connection from conductors to ground in an ungrounded system takes place through total system parasitic capacitance to ground. The main advantage of ungrounded systems is their ability to operate through a single conductor to ground fault. Because ground fault current is minimal, another advantage of these systems is that little or no damage occurs at the point of failure. However, high impedance to ground turns into a disadvantage during intermittent ground faults that produce under-damped oscillatory overvoltages with respect to ground and may cause insulation damage or breakdown. In the case of a DC system, the slightest unbalance in leakage currents from individual conductors will result in a voltage unbalance between individual conductor-to-bus voltages.
A solidly grounded system connects the neutral point of the source to ground. Because fault current is limited only by the source and ground impedances, a single ground fault may result in significant damage at the point of failure. The fault current may also generate hazardous voltage at the point of failure and at any point in the ground return path.
In low resistance grounding systems the source neutral is connected to ground through a resistor sized to limit the ground fault current to approximately full load current. Although such systems limit transient overvoltages and damp oscillations, they do not allow operating under ground fault conditions for long periods of time.
A reactance-grounded system connects a reactance between the system neutral and ground. This method is employed for neutral-grounded generators that are grounded through a low-value reactor to keep the ground-fault current below the three-phase fault current of the generator.
A high resistance-grounded system connects the neutral point of the source to ground through a resistor. In a high resistance grounding system, a purposely inserted grounding resistance would limit the ground fault current such that the current can flow for an extended period of time without exacerbating damage. The grounding resistor reduces fault currents to a safe level and eliminates potential damage and safety hazards of ungrounded systems. For some types of ground faults this approach may reduce transient overvoltage and oscillation because a grounding resistor provides damping for the equivalent system LC circuit.